1. Field of the Invention
The present invention relates generally to drilling a subterranean wellbore and, more specifically, to sensors used in connection with nested tubular assemblies that can drill and line a section of a wellbore without having an intervening trip of a drill string and BHA to the surface.
2. State of the Prior Art
Hydrocarbons such as oil or gas from an oilfield are produced from wellbores intersecting one or more hydrocarbon producing reservoirs in the oilfield. The time and capital investment associated with drilling such wellbores have always been substantial. Factors influencing the overall cost of a well include the time required to drill a wellbore, the geographical accessibility of the oil field, and the complexity and/or depth of the wellbore. In the discussion below, it will become apparent that under many circumstances, the predicted costs for drilling a particular wellbore cannot be sufficiently offset by the expected production of hydrocarbons from the reservoir the wellbore drains, thereby making such oilfields uneconomical to develop.
As is well known, oilfield wellbores are drilled by rotating a drill bit conveyed into the wellbore by a drill string. The drill string includes a drill pipe (tubing) that has at its bottom end a drilling assembly (also referred to as the “bottomhole assembly” or “BHA”) that carries the drill bit for drilling the wellbore. After a selected portion of the wellbore has been drilled, this “open hole” section is usually lined or cased with a string or section of casing. In some cases, it may be possible to drill a wellbore to the target depth and thereafter case the wellbore. More frequently, the planned trajectory of a wellbore and formation properties will require sections of the wellbore to be cased before successive sections of the wellbore can be drilled. For instance, the wellbore may intersect a number of zones, each of which may have different fluids (e.g., water, gas, oil). Thus, casing may be needed to provide zonal isolation; e.g., prevent a water zone from invading an oil zone. Moreover, the drilling activity may require the use of drilling fluid having pressures that exceed the fracture pressure of the “open hole” sections. Thus, the casing may be needed to prevent damage to the exposed formation. Also, the casing may be needed to maintain wellbore stability; e.g., to prevent the wellbore from collapsing. Therefore, drilling and casing according to the conventional process typically requires drilling a section of the wellbore, tripping the drill string and drill bit out of the wellbore, conveying a casing into the wellbore, cementing the casing in place, tripping the drilling string back into the hole, drilling the next section of the wellbore, and so on.
Unfortunately, conventional drilling and casing methods can be time consuming because wellbores routinely reach depths of thousands of feet. Thus, the time required to simply trip the drill string into and out of the wellbore can require dozens of hours. During tripping, no other meaningful activity usually occurs (e.g., drilling or casing the wellbore). This idle time can be particularly disadvantageous given that rig costs can approach and exceed one hundred thousand dollars per day. Multiple trips also are disadvantageous because they can delay the beginning of profitable production. Moreover, control of the well may be difficult during the period of time that the drill pipe is being removed and the casing is being disposed into the wellbore. Also, as is known, each trip into and out of the wellbore carries the risk that the drill string may become stuck in the wellbore or suffer some other time of failure that requires an expensive remedial operation (e.g., fishing, sidetrack, etc.).
The present invention addresses these and other drawbacks of the prior art.